Different types of information can be digitally represented and stored on optically read discs. Compact Disc (CD) drives and Digital Versatile Disc (DVD) drives may be used to read the information from the discs. Examples of media that may be read using a CD or DVD drive are Compact Disc-Digital Audio (CD-DA), Compact Disc-Read Only Memory (CD-ROM), Compact Disc-Recordable (CD-R), Compact Disc-ReWritable (CD-R/RW and CD-RW), and Digital Versatile Disc-Read Only Memory (DVD-ROM).
While all CD formats define sectors having a length of 2352 bytes, the composition of information contained in the sectors differs depending on the format. The data size may be less than the full 2352 bytes if other information, such as error checking or correction coding, is included within the sector. CD-DA discs contain digitally encoded audio information organized into sectors containing 2352 bytes of data without sector level error checking or correction capabilities. In comparison, CD-ROM discs contain a variety of digital computer data including text, data records, and multimedia information such as graphics, audio, and video. Information on CD-ROM discs may be organized according to a variety of different sector formats, as indicated by a sector's MODE and FORM data fields.
For example, a standard (Mode-1) CD-ROM sector contains 2048 bytes of data, but a multimedia (Mode-2, Form-2) CD-ROM/XA sector contains 2324 bytes of data. There are three modes defined for CD-ROM. These are Mode-0, Mode-1, and Mode-2. The currently defined Modes in the CD-ROM specification are shown below in Table 1.
TABLE 1 Mode Data Size Characteristics Mode-0 2336 Bytes Contains all zeros in the data field Mode-1 2048 Bytes Contains real data in addition to error-detection and correction capabilities Mode-2 No Form 2336 Bytes Form-1 2048 Bytes Contents further depend on the Form Field Form-2 2324 Bytes
A Mode-2 sector format may further be one of several defined forms: "No Form," Form-1, or Form-2. The currently defined forms for a Mode-2 CD-ROM sector and the available features are shown below in Table 2.
TABLE 2 Subheader Sync- & MODE hro- Duplicate Error Error 2 nization Header Subheader Data Detection Correction FORM Field Field Field Field Field Field No Form Yes Yes No Yes No No Form-1 Yes Yes Yes Yes Yes Yes Form-2 Yes Yes Yes Yes No No
As seen in the tables above, error detection and correction depend on the sector type and format of the CD-ROM. While CD-DA discs typically contain no sector level error detection or correction, a CD-ROM may or may not contain error detection and/or correction information depending on the sector format indicated by the mode and form fields.
Systems capable of reading different formats of optical discs must be able to differentiate between the formats to properly read and access the stored information. Typically, software or firmware has been used to examine the mode and form fields of the sector format to determine the format of a CD-ROM. These software and firmware techniques require a microcontroller or microprocessor to frequently access the stored information to determine its format. Accessing the sector fields to determine the mode and form causes delays that may result in bottlenecks for the drive. This is especially true as the speed of CD-ROM data reads becomes faster. In the early 1990's CD-ROM's were operating at up to the quad-speed (4.times.) level. In 1998, it is not uncommon to find CD-ROM's operating at 32.times. and beyond. As speeds have increased, challenges have arisen in the area of error detection and correction, as well as in other areas dependant on control of the CD sectors being serially transferred from the disc to the buffer.
As CD-ROM speeds increase, there is an increased need for fast buffer accesses. In order to effectively stream data from the optical disc to the host computer or device, it becomes desirable to reduce the delays associated with frequent microprocessor or microcontroller accesses. Needed is a device to determine the information storage format that minimizes the impact on system resources.
Economic pressures are making it more desirable to use less expensive, lower performance microprocessors and microcontroller. When used with existing format detection and selection systems, these lower performance devices make it more difficult to achieve high data access speeds.
Hardware has been used to automate and streamline a limited portion of the format identification process, such as detecting the Q-Channel CD-ROM bit to determine whether the current sector is a CD-ROM sector. However, it would be desirable to use logic circuitry in conjunction with other hardware to automate the entire process, including identification of mode and form, in order to improve access speeds while allowing the use of less expensive, lower performance microprocessors and microcontrollers.